1. Field of the Invention
The invention relates to a rack shaft supporting device and a steering system using the rack shaft supporting device.
2. Description of Related Art
As a rack shaft supporting device, a support yoke clearance automatic adjusting device is described in US 2008/0295629 A1. The support yoke clearance automatic adjusting device includes a support yoke by which a rack shaft is slidably supported, a cam that supports the support yoke, a support plug that supports the cam and that is connected to a rack housing, and a yoke spring that applies an elastic force to the support yoke in a direction toward the rack shaft. Because the rack shaft is pushed against a pinion by the elastic force, it is possible to suppress generation of abnormal noise or the like at meshed portions of the rack shaft and the pinion.
On an opposed face of the cam, which is opposed to the support yoke, three chevron separation portions that project toward the support yoke are formed in respective three regions each of which corresponds to 120° and which are located next to each other in the circumferential direction. Each of the separation portions has a slant face gradually slanted toward the support yoke, and an end face that extends, in a direction away from the support yoke, from a support yoke-side edge of the slant face.
Three portions corresponding to the separation portions of the cam are formed at a portion of the support yoke, which is opposed to the cam, so that the separation portions of the cam are engaged with the separation portions of the support yoke (the portions corresponding to the separation portions of the cam). On the end face of each of the separation portions of the cam, there is provided an elastic member that projects toward the adjacent separation portion of the support yoke (the portion corresponding to the separation portion of the cam). When the support yoke is abraded by friction or the like with the rack shaft, or when rack teeth of the rack shaft are abraded by the mesh between the rack shaft and the pinion, the cam rotates relative to the support yoke due to an elastic force of the elastic member, and each of the separation portions of the cam pushes the support yoke toward the rack shaft. Thus, the support yoke is pushed toward the rack shaft irreversibly by an amount corresponding to the abrasion and the abrasion is compensated for, so that the yoke spring is able to apply a constant elastic force to the support yoke in spite of the abrasion.
In the support yoke clearance automatic adjusting device described in US 2008/0295629 A1, when the cam rotates relative to the support yoke, the separation portions of the cam and the separation portions of the support yoke (the portions corresponding to the separation portions of the cam) make surface contact with each other and slide relative to each other. Thus, significantly high frictional resistance to the rotation of the cam is generated. This causes a possibility that even if the abrasion occurs, the cam is not able to rotate promptly in response to the abrasion and therefore the abrasion is not sufficiently compensated for.
In addition, on the opposed face of the cam, which is opposed to the support yoke, the separation portions of the cam are formed in the respective three regions each of which corresponds to 120° and which are located next to each other in the circumferential direction. Therefore, it is possible to compensate for abrasion by moving the support yoke toward the rack shaft, while the cam is rotated one-third turn. However, it is not possible to compensate for an abrasion larger than the abrasion that is compensated for as described above. Further, in some cases, high-viscosity grease is provided between the cam and the support yoke in order to prevent, when the rack shaft is distorted by an external force or the like and moved away from the support yoke suddenly, the support yoke from suddenly moving toward the rack shaft so as to follow the bending of the rack shaft. As a result, the support yoke is prevented from locking the movement of the rack shaft. Because the support yoke is connected to the cam via the grease, it is possible to prevent occurrence of a problem that only the support yoke suddenly moves toward the rack shaft. However, characteristics of the grease tend to vary with temperature, which makes it difficult to reliably prevent the support yoke from suddenly moving toward the rack shaft. Further, if high-viscosity grease is provided between the cam and the support yoke, one of the cam and the support yoke comes off the other, and therefore the relative position between the cam and the support yoke may be offset from the proper relative position. In view of this, preferably, grease should not be provided between the cam and the support yoke.